If a wall looks wrong, the art is usually too small, so when you're torn between two sizes, take the larger one. On an open wall, hang the piece so its centre sits about 145 cm from the floor, which is average eye height and the rule galleries work to. When furniture sets the height, the furniture wins: above a sofa or console, aim for the artwork to run about two thirds of its width and hang it 15 to 20 cm above the back, close enough that the two read as one composition.
Groupings follow the same logic scaled up. Treat the set as a single shape, keep the gaps consistent at around 5 cm, and lay the arrangement out on the floor first, because moving paper templates costs nothing and moving hooks costs your afternoon and your wall.
On handling, a stretched canvas is light and forgiving, happy on a single decent hook, and its texture rewards light raking across it. A framed print arrives ready to hang, glazed in museum-grade plexiglass, so match the fixing to your wall type and hang it where lamps and windows won't sit in the reflection. Either way measure twice, because the piece will hang there for years and the extra five minutes shows for all of them.